Simple G.711 pass-through has proven to be an inadequate method of transmitting data-modem traffic over packet networks that experience packet loss. The loss of packets in a communication disrupts the transmitting modem's signal, causing the connection to fail and reducing the data throughput of the channel. This is a real and growing problem in the current Voice-over-Internet Protocol (VoIP) market.
One possible remedy to alleviate the problem would be to incorporate Forward Error Correction (FEC) techniques to help recover G.711 data lost over the network. The simplest FEC technique to employ would be data redundancy, whereby each network packet contains data from previous network packets in addition to its own data. This approach offers a low implementation complexity at the cost of more network bandwidth and is an attractive approach for high-density systems.
An alternative approach is to terminate locally all modem signals at each gateway and exchange demodulated data between the gateways over the packet network using a reliable packet transport protocol. FIG. 1 illustrates an example of this Modem over IP (MoIP) concept currently being implemented.
A major challenge to deploying any successful MoIP implementation requires a robust modem/facsimile call discrimination mechanism which must be capable of detecting, identifying and discriminating between modem and facsimile signals so that the proper processing can be activated at each gateway. The detection, classification and change of processing must be done in such a way as not to cause interoperability problems which can result in unacceptable high modem/facsimile call failure rates or voice quality degradation.
To overcome the drawbacks of the prior art, a new channel state, called the link-layer modem relay (LLMR), is added to the DSP, although it is common to use a DSP, use of other processors or formats is within the scope of the present invention. The LLMR is responsible for link-layer modem relay processing. FIG. 2A illustrates the DSP channel architecture for a LLMR channel state. In this state, pulse code modulation (PCM) samples are received by the PCM interface unit 7 and processed by the Data Modem Pump (DMP) 8. The DMP implements V-series voice-band data modem processing and demodulates the incoming data stream. The modem packetized protocol (MPP) 9 performs high-level data link control (HDLC) processing on the received data and passes the decoded HDLC data to the Link-layer Modem Relay Unit (LLMRU) 10. The LLMRU 10 performs local V.42 processing and packetizes the data for transmission over the packet network. Likewise, modem data received from the packet network is converted to a V.42 data stream and passed to the MPP 9 for modulation by the DMP 8.
Similarly, as illustrated in FIG. 2B, a facsimile channel state is used for handling facsimile traffic over packet networks. The facsimile modem (FM) pump implements V-series voice-band facsimile modem processing and facsimile interface unit (FIU) packets, the facsimile protocol communication in a format suitable for transmission over a packet network.